Glassy dynamics of soft matter under 1D confinement: how irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films.

The structural dynamics of polymers and simple liquids confined at the nanometer scale has been intensively investigated in the last two decades in order to test the validity of theories on the glass transition predicting a characteristic length scale of a few nanometers. Although this goal has not yet been reached, the anomalous behavior displayed by some systems--e.g. thin films of polystyrene exhibit reductions of Tg exceeding 70 K and a tremendous increase in the elastic modulus--has attracted a broad community of researchers, and provided astonishing advancement of both theoretical and experimental soft matter physics. 1D confinement is achieved in thin films, which are commonly treated as systems at thermodynamic equilibrium where free surfaces and solid interfaces introduce monotonous mobility gradients, extending for several molecular sizes. Limiting the discussion to finite-size and interfacial effects implies that film thickness and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. On the contrary, such an oversimplified picture, although intuitive, cannot explain phenomena like the enhancement of segmental mobility in proximity of an adsorbing interface, or the presence of long-lasting metastable states in the liquid state. Based on our recent work, we propose a new picture on the dynamics of soft matter confined in ultrathin films, focusing on non-equilibrium and on the impact of irreversibly chain adsorption on the structural relaxation. We describe the enhancement of dynamics in terms of the excess in interfacial free volume, originating from packing frustration in the adsorbed layer (Guiselin brush) at t(*) ≪ 1, where t(*) is the ratio between the annealing time and the time scale of adsorption. Prolonged annealing at times exceeding the reptation time (usually t(*) ≫ 1 induces densification, and thus reduces the deviation from bulk behavior. In this Colloquium, after reviewing the experimental approaches permitting to investigate the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy, we propose several methods to determine gradients of mobility in thin films, and then discuss on the unexploited potential of analyses based on the time, temperature and thickness dependence of the orientational polarization via the dielectric strength.

Comparing the Effect of Product-Based Metrics on the Translation Process.

Characteristics of the translation product are often used in translation process research as predictors for cognitive load, and by extension translation difficulty. In the last decade, user-activity information such as eye-tracking data has been increasingly employed as an experimental tool for that purpose. In this paper, we take a similar approach. We look for significant effects that different predictors may have on three different eye-tracking measures: First Fixation Duration (duration of first fixation on a token), Eye-Key Span (duration between first fixation on a token and the first keystroke contributing to its translation), and Total Reading Time on source tokens (sum of fixations on a token). As predictors we make use of a set of established metrics involving (lexico)semantics and word order, while also investigating the effect of more recent ones concerning syntax, semantics or both. Our results show a, particularly late, positive effect of many of the proposed predictors, suggesting that both fine-grained metrics of syntactic phenomena (such as word reordering) as well as coarse-grained ones (encapsulating both syntactic and semantic information) contribute to translation difficulties. The effect on especially late measures may indicate that the linguistic phenomena that our metrics capture (e.g., word reordering) are resolved in later stages during cognitive processing such as problem-solving and revision.

Crystallization of thin polymer layers confined between two adsorbing walls.

Confined at the nanoscale level, polymers crystallize much slower than in bulk, and in some cases, the formation of ordered structure is inhibited within extremely long experimental time scales. Here, we report on the thickness dependence of the conversion rate of the amorphous fraction of ultrathin films of poly(ethylene terephthalate) during isothermal cold crystallization. We present a new analytical method assessing the impact of irreversible chain adsorption and permitting to disentangle finite size and interfacial effects. From the µm range down to a few tens of nm, we observed an increase in crystallization time scaling with the inverse of the film thickness, which is a fingerprint of finite size effects. Films thinner than ∼20 nm did not crystallize, even after prolonged annealing in the temperature range where the crystallization rate reaches its maximum value. Noticing that this threshold corresponds to the total thickness of the layer irreversibly adsorbed within our investigation time, we explain these findings considering that chain adsorption increases the entropic barrier required for the formation of crystalline structures.